Frequently, it is desirable to provide a substrate with a covering which will protect it from a corrosive or otherwise hostile environment. Sleeves of thin walled polymeric material have proven to be useful for this purpose, particularly when the article has tubular or otherwise more or less regular elongate configuration. Particularly useful are sleeves fabricated from heat recoverable material. Such a sleeve can be constructed with a diameter large enough so that it can be easily positioned about a substrate. Then, owing to its ability to shrink to a smaller diameter when heated, the sleeve can be heat recovered so that it closely conforms to the substrate.
Heat recoverable materials suitable for making such sleeves are well known. For example, in Cook U.S. Pat. No. 3,086,242, the disclosure of which is incorporated by reference, a heat rcoverable material obtained from a radiation crosslinked polymeric material is described. Heat recoverable sleeves made from such a material have been found to be particularly useful as corrosion resistant coverings for the joints of large diameter pipes, for example those used for pipelines for the transmission of oil, gas or other fluids.
Sleeves of small diameter can be conveniently extruded but this is not the case with sleeves of a large enough diameter to be employed on large pipes. Therefore, the sleeves heretofore employed for such pipes have typically been of the "wrap around" type. Such sleeves are made by wrapping a length of heat recoverable material about the pipe or other substrate and joining its end edges by mechanical or other suitable means that prevent the ends from separating when the sleeve is recovered. Examples of such sleeves are described in the following U.S. Patents, the disclosures of which are incorporated by reference: Conde, U.S. Pat. No. 3,379,218, Ellis, U.S. Pat. No. 3,455,336, Wilson, U.S. Pat. No. 3,530,898 and Evans, U.S. Pat. No. 3,770,556.
To facilitate the installation of heat recoverable sleeves at the job site, it would be advantageous to have available a supply of prefabricated sleeves of large diameter. To accomplish that end, it has been proposed to form a sleeve by joining opposing edges of a sheet of polymeric material capable of having heat recoverability imparted thereto. The sleeve formed in this way is then expanded to the desired heat recoverable diameter by known methods. Such prior art sleeves are described in Naidoff, Ser. No. 436,674 filed Jan. 25, 1974 and Humphries, Ser. No. 476,895 filed June 6, 1974. These applications have the same assignee as the present invention and their disclosures are incorporated by reference. The pre-expanded sleeves described in those applications can be made by inserting the ends of the polymeric sheets into the channels of one or more generally I-shaped inserts fabricated from thermoplastic material that contains a cross-linking agent followed by heating the assembly under pressure to affect a weld.
A drawback to the aforementioned sleeves is that the process by which they are made heat recoverable is a slow one making the fabrication of a large number of sleeves impractical. One such process and the apparatus employed in its execution is described in Greuel et al., Ser. No. 436,675 filed Jan. 25, 1974, an application having the same assignee as the present invention, the disclosure of which is incorporated by reference.
By contrast, sheet material already having the property of being heat recoverable can be made at a high rate of speed. Therefore, it would be advantageous if large diameter heat recoverable sleeves could be fabricated directly from a sheet of preformed heat recoverable material. Prior to this invention, direct formation of a heat recoverable sleeve has been accomplished by bringing opposing edges of a length of heat recoverable material into an abutting relationship and overlaying the bond line with a length of a laminate comprising a glass cloth sandwiched between layers of cross-linked polyethylene. A layer of cross-linking agent such as t-butylperbenzoate is disposed between the laminate and the heat recoverable material. The resulting assemblage is heated under pressure to activate the crosslinking agent and thereby chemically bond the sleeve material to the laminate. Although having valuable applications, this type of sleeve is handicapped in that if after recovery about a substrate there remains a high level of "unresolved recovery", i.e., above about 70%, the sleeve fails at the bond. For the purposes of this application the percent of unresolved recovery is determined by the following formula: EQU U - (S - R/R)
in that formula, U is the percent of unresolved recovery (which may be more than 100), S is the diameter of the substrate about which the sleeve is to be recovered and R is the diameter to which the sleeve would recover if unrestricted. Bond failure with this sleeve occurs by the detachment of the recovered sleeve material from the laminate.
If it is attempted to make a heat recoverable sleeve directly from heat recoverable material using the inserts discussed by Naidoff or Humphries, the weld that is formed is frequently unsatisfactory, as is contains voids and other weak spots. This results because the heat employed in the weld forming process causes the ends of the heat recoverable sheet to begin to recover and withdraw from the insert. To date, therefore, the need for a method of directly forming large sleeves from heat recoverable materials in a manner that the sleeve is capable of tolerating a high percentage of unresolved recovery without failure at the bond line remains unsatisfied.
Accordingly, it is an object of this invention to provide an improved heat recoverable sleeve.
It is another object of this invention to provide a method for making sleeves directly from heat recoverable material capable of tolerating high levels of unresolved recovery.